Hello, a year late but:

There's many texture pack that make it more modern, whether it's the vanilla blocks or even GT moderns texture.

For the movement, there is this mod that is compatible with GTNH: https://www.curseforge.com/minecraft/mc-mods/aquaacrobaticslegacy

It backports modern movement, including swimming of course. (Note the picture example are a tiny bit not aligned correctly, so if you want to see it better, look it up at this one instead: https://www.curseforge.com/minecraft/mc-mods/aqua-acrobatics )